John Rumm formulated the question :
IME many of the Vaillant boilers do support split temperature operation, but
it does depends on what controls you have them paired with.

The 400 series Vaillant I did (probably previous model range, since this was
= 12 years ago), actually had separate knobs on the front for CH and DHW
flow temperatures. Although that was retrofitted into system with
traditional controls that could not distinguish between the source of the
call for heat - hence the DHW control never came into play and it ran like
your system does.

The blurb on their web site:

https://www.vaillant.co.uk/for-insta...ler-26116.html

Seems to suggest its compatible with the VRC 700 weather compensating
controls, so that ought to allow split temperature as well.

I have studied it and not found any obvious way to split the
temperature. I even spoke to Vailant and they confirmed there was no
way it could be done.